As the material having shape memorizing characteristics, shape memorizing alloys have been already known. Examples of this type may include Cu--Al--Ni alloys, Au--Cd alloys, In--Ti alloys, Ni--Ti alloys, etc. Although these shape memorizing alloys have shape memorizing characteristics with excellent heat-sensitivity, they have not yet been widely used except for special uses, because their base material is very expensive or the heat treatment or processing thereof for exhibiting shape memorizing characteristics is not necessarily easy.
On the other hand, several types of polymers have been already known as polymers having heat-sensitive shape memorizing performance These can be classified by structure into, crosslinked products of polymers having an adequate melting point or a glass transition temperature exceeding normal temperature or cold processed products of polymers having an adequate melting point or a glass transition temperature exceeding normal temperature and having a remarkably high molecular weight.
Generally, in a temperature range of not higher than the glass transition temperature or the melting point, a polymeric material shows restricted thermal motion of its molecular chains and exhibits properties as a rigid resin. However, when it is heated to not lower than the glass transition temperature or the melting point, it becomes a so-called rubbery substance. This type of temperature dependency is common in all of the polymeric materials. Although there are many aspects to be considered such as the temperature range wherein a glass transition temperature or a melting point exists, the readiness in plastic deformation, etc., most polymeric materials having some substantial crosslinking points to the extent that the strain is not relaxed, have shape memorizing properties to some extent.
More specifically, a molded resin product of a certain type of polymer is prepared by various molding methods, and after molding, a crosslinking reaction is carried out for memorizing the shape. When the molded product is elevated to a temperature not lower than its glass transition temperature or melting point, at which deformation occurs, and then is lowered to a temperature which is not higher than the glass transition temperature or the melting point in the state at which deformation has occurred, its strain is maintained. This is because the thermal motion of molecular chains is restricted at a temperature not higher than the glass transition temperature or melting point, whereby the strain is fixed. When the deformed molded product is again heated to a temperature not lower than the glass transition temperature or melting point wherein the thermal motion of molecular chains is possible, the strain is released to restore its original shape.
As such a shape memorizing resin, a crosslinked product of a crystalline polyolefin (U.S. Pat. No. 3,086,242), a crosslinked product of crystalline trans-polyisoprene (Japanese Unexamined Patent Publication No. 16956/1986), a crosslinked product of crystalline trans-polybutadiene (U.S. Pat. No. 3,139,468), etc. have been known. Among polyolefins, particularly a crosslinked product of crystalline polyethylene has been practically applied for uses such as heat shrinkable tube, etc. However, in these crystalline polymers, in order to prevent the inhibition of crystallization by crosslinking, an operation for exhibiting shape memorizing characteristics is required such that the crosslinking is required to be effected by vulcanization at a low temperature or irradiation of radiation in the state in which the polymer is crystallized, etc. Therefore, these types of shape memorizing resins have not yet been widely used except for specific uses.
Further, when the polymer has a remarkably high molecular weight, even at a temperature not lower than the glass transition temperature, the entanglement of the polymer molecular chains becomes substantially the cross-linking points and hence the strain is not relaxed, whereby the polymer exhibits shape memorizing function. As examples of these types of shape memorizable resins, polynorbornene (U.S. Pat. No. 4,831,094), polyvinyl chloride, polymethyl methacrylate, polycarbonate, acrylonitrile-butadiene resin, etc., are known.
However, to ensure that this type of polymer exhibits a sufficient shape memorizing function, the molecular weight must be made remarkably high (e.g. 2,000,000 or higher), and in this case, flowability of the polymer will be necessarily lowered to a great extent which renders processing by a general purpose plastic processing machine such as injection molding, extrusion molding, etc. extremely difficult. Also, there is a technique, in which the molecular weight is set at a slightly lower level than described above and cold processing is effected, by deforming at a temperature not higher than the glass transition temperature, but this technique requires special operations which make the processing production steps complicated, having difficult problems such as insufficient restoration performance, etc. Therefore, it has not been widely used either.
As a new technical principle to improve the problems in processability which are inherent in the prior art of using these polymers, a shape memory resin utilizing a block copolymer has also been already developed. Examples of this technique include the technique of using a fluorine resin type block copolymer (Japanese Unexamined Patent Publication No. 227437/1984), the technique of using a polyester, a polyether or a polyurethane type block copolymer (International Publication No. WO 86/03980) and the technique of using a crystalline styrene-butadiene type block copolymer (European Patent Publication No. 0234512).
These types of techniques, while generally having accomplished improved processability, still have respective remaining problems. In the technique of using the fluorine resin type block copolymer as described above, although it having specific features in flame retardancy, heat aging resistance of the molded product obtained, for exhibiting sufficiently shape memorizing performance, a crosslinking by electron beam irradiation is required, and it still has a problem in processing production steps. On the other hand, the technique of using a polyester, a polyether or a polyurethane type block copolymer as described above is inferior in heat resistant stability, weathering resistance as performances of the base material to be used for industrial uses. Further, the technique of employing the above-described crystalline styrene-butadiene type block copolymer provides a material having sufficient process-ability by means of a general purpose plastic processing machine, which has accomplished excellent shape memorizing performance. However, since this polymer contains many unsaturated bonds in the polymer chain, when industrial uses are considered, problems remain with respect to heat resistant stability and weathering resistance.
On the other hand, hydrogenated products of styrene-butadiene block copolymers have been already known. This type of polymer is a hydrogenated product containing generally 35 to 55% of 1,2-linkage in the butadiene linkage chains, and has excellent performances as noncrystalline thermolastic elastomer (U.S. Pat. No. 3,431,323). However, neither performance nor utilization of this kind of base material as a shape memorizing resin have yet been known in the prior art.
The present invention intends to solve the various drawbacks described above as observed in the shape memory materials of the prior art, namely the problem of cumbersome handling as a result of an application of general purpose plastic processing method such as injection molding, extrusion molding, etc. being generally difficult and a special operation, being required such as a crosslinking reaction at a low temperature.
As a result, the present invention provides a shape memory polymer resin, a resin composition and a shape memorizing molded product, which are also excellent in performance as the resin material such as not only heat resistance and weathering resistance, but also in strength, etc.
The present inventors have investigated intensively in order to develop a shape memorizable resin and a resin composition which solve these problems of the prior art, and consequently have found that a molded product obtained by molding a copolymer resin having a specific structure or a resin composition containing the same copolymer resin as the component by a generally used molding machine of plastics such as an injection molding or extrusion molding machine and then remolding it to a form different from the molded shape in the specified condition, has extremely excellent shape memorizing performance without requiring any special operation for imparting a shape such as crosslinking reaction, etc., and as a result, have accomplished the present invention.